Wave length modulation



July 13, 1943. M. G. CROSBY WAVE LENGTH MODULATION Filed May 8, 1941 2 Sheets-Sheet l a 4 a I r Maw INVENTOR Mar/2g (Z U'm-Zg ATTC, RNEY July 13, 1943. M. Gw CROSBY WAVE LENGTH MODULATION Filed May 8, 1941 2 Sheets-Sheet 2 INVENTOR W ATTORNEY Patented July 13, 1943 WAVE LENGTH MODULATION Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 8, 1941, Serial No. 392,421

2 Claims.

This application concerns a new and improved wave length modulator of the type disclosed in my United States applications Serial No. 209,919, filed May 25, 1938, now U. S. Patent No. 2,250,095, dated July 22, 1941, Serial No. 311,047, filed December 27, 1939, and Serial No. 312,446, filed January 5, 1940, now U. S. Patent No. 2,279,660, dated April 14, 1942. In systems of this type push-pull reactance tubes are used to frequency modulate the tuning of an oscillator or phase modulate the tuning of an amplifier in accordance with modulating potentials which have or have not been subjected to preemphasis or relative amplitude correction in a desired manner. The new principle disclosed here is the use of like feedback phase shifters for the two reactance tubes. The reactive effects of the two tubes are made opposite by feeding the phase shifters from opposite sides of the tuned circuit. The use of like phase shifting in the circuits of the reactance tubes and wave generator simplifies -arrangement, tuning, and adjustment thereof.

This type of modulator has all of the advantages which are obtained by push-pull reactancetube modulators. These advantages include elimination of reaction from element voltage variations, increased linearity of modulation, and increased range of modulation. In addition, the present modulator has the advantage that the balance of the reactance tubes is more easily obtained. This ease of balance is obtained by the fact that, for some circuits, identical phase shifters may be used, while for other circuits the relative adjustments of the phase shifters is quite simple.

In describing my invention in detail reference will be made to the attached drawings: Figure 1 illustrates the essential elements of a wave length modulator using a pair of reactance tubes with similar phase shifting circuits; Figure 1a is a modification of the arrangement of Figure 1; while Figures 2, 3 and 4 are modifications of the arrangements of the prior figures.

The circuit of Figure 1 shows an embodiment using the principle of this invention. The principle concerns the manner of obtaining the phase shifted voltage which is fed back to the grids of the reactance tubes. In this arrangement 4 is the oscillator tube and 6 and '8 are the reactance tubes. The oscillator includes a tuned circuit l connected at one end to the anode l2 of tube 4 and at the other end to the'grid l4 by way of a coupling and blocking condenser CB. Bias is supplied by resistance IS. The cathode i8 is coupled to a point on the inductance of It,

Note that this point is closer to the grid end of circuit I0 than to the anode end thereof. The anodes 20 and 22 of tubes 6 and 8 are coupled to one end of circuit l0. Grids 24 and 26 of tubes 6 and 8 are coupled by condensers and reactance combinations C and R and C' and R to the ends of circuit 10. CgK and CgK' represent the grid to cathode capacity of tubes 6 and 8 respectively and are taken into consideration in deriving the phase quadrature voltages on the anodes and grids of tubes 6 and 8. Bias is supplied to the grids 24 and 26 by resistances 28 and 30 and 32 and 30. Modulating potentials are applied by a transformer T to the grids 34 and 36 in pushpull relation.

The anodes 20 and 22 are excited by generated voltages of like phase. Grid 24 has a generated voltage of a phase which lags the voltage on the anode 20 by about degrees so that tube 6 is in effect a variable inductance on the generator circuits. The voltage on the grid 26 is about degrees out of phase with respect to the voltage on grid 24 and so leads the voltage on the anode 22 by about 90 degrees. Tube 8 is in efiect a variable capacity in shunt to the generator. Differential control of the gain of tubes 6 and 8 increases the inductive and capacitive effects when the gain increases and vice versa to thereby control the tune of the generator in accordance with the modulating potentials.

In the push-pull reactance-tube circuits of my prior dockets one reactance tube received a 90 degree phase shift by means of a resistancecapacity phase shifter and the other reactance tube receives a 270 degree phase shift by means of a capacity-resistance phase shifter. In the modulator of the present application as described above like phase shifters are used for both reactance tubes, and the 180 degree phase difference between them is obtained by feeding the two phase shifters from opposite ends of the tuned circuit of the oscillator.

In Figure 1, tubes 6 and 8 are the reactance tubes and tube 4 is the oscillator. The reactancetubes are of the type which has an extra grid available for the application of the modulation. The modulation is applied to the extra grid and the phase-shifted feedback voltage is fed to the control grid. The phase shifter for tube 6 consists of resistance R and the grid-to-cathode capacity Cgk. C is a blocking condenser to keep the plate voltage off the control grid. The phase shifter for tube 8 consists of resistance R and Cgk. C is likewise a blocking condenser. It will be noted that the phase shifter for tube 6 2 assess:

is fed from the plate side of tuned circuit II while the phase shifter for tube I is fed from the opposite end or grid side of the coil II. Bince the grounded point on the coil of tuned circuit II is at, or near, the middle of the coil, the voltage Where the grounded point on the coil of tuned circuit 4 is not at the mid point of the coil, the feedback voltage for they two reactance tubes will not be balanced if the phase shifters employ identical elements. This unbalanced condition may be remedied by an adjustment of the relative magnitudes of resistors R and -R' or condensers Cole and. Cole. For instance, in the case shown in Figure l, the tap II on the coil II of' tuned circuit II is offset towards the grid end of the coil 38. This tends to reduce the feedback voltage of reactance tube I as compared to reactance tube I when equal phase shifters are used. One method of rebalancing the feedbacks is to increase the value of resistance R; another is to increase the capacity of Cal: by adding external capacity. A still further method is to feed the phase shifter for tube I from a tap I! on the coil II as shown in Figure 1a. In this case balance is attained by varying the position of tap 42.

Another method of applying the principle of this invention is shown in Figure 2. In i: his

shunt inductance, since the generated voltage 45 on its grid lags the generated voltage on its anode about 90 degrees, while reactance tube I has a phase shift between its plate and grid circuits which is 180 degrees displaced from the relative phase shift obtained in tube I. The generated voltage on the grid 2I of reactance tube 8 is-the same as the generated voltage on the grid 24 and. leads the voltage on the anode 22, This causes reactance tube] to act as a shunt capacity so that as in Figures 1 and la push-pull modulation is required from transformer T. In this circuit of Figure 2 a still fur- I ther means of balancing the oscillator voltage fed to the two tubes is employed. In this modiflcation the coil II of the oscillator circuit I is midtapped. This gives the proper balance for the reactance tubes, but introduces too much regeneration in the oscillator. This excess regeneration is comprensated for by inverse feedback which is introduced by means of unbypassed cathode resistor Rk.

It will be apparent to those skilled in the art that tuned circuit II may consist of the tuned circuit of a push-pull amplifier. This amplifier then has its tuning modulated by the reactance tubes so that phase modulation of the carrier amplified by the amplifier is accomplished.

It will also be apparent that other types of phase shifters such as the resistance-inductance, RILI, type of Figure 3 and the capacityresistance. 02,32, type of-Figure4csnbeused for the phase shifters in the application of the principle of thisinventionl. v V a Figure SissimilartoligureQiht-hatthegrids s and 2Iareconnectedtothesamepointon the tank circuit I. while the-anodes II "16,22;

are connected to points at which the voltages are of substantially opposed phases. In this modification 0 and C are again blocking condensers.

The phase shifting reactances are RILI for tube I and RI'L' for tube I. In Figure 2-tube I acts as an etlective inductance since. the generated voltage on its grid 24 lags the generated voltage on its anode about 90 degrees. In Figure 3 phase shifters RILI cause the generated votage on the grid 24 m lead the generated voltage on its anode and as a consequence tube \I in Figure 3 acts as a shunt capacity. The generated voltage in the grid 2I in Figure 3 is sub- 20 .stantiallythe same as that onwgrid I, and since the voltage on the anode 22 is about I80 degrees out of phase with respect to the voltage on the anode 20 the voltage on grid 2I lags the voltage on anode 22 and tube I acts as a shunt induct- The similarity between the arrangement of Figure land the modification in Figure 4 will be apparent by inspection. In Figure l C and C are blocking condensers. The phase shifters R and Cox cause tube I to act as a shunt incapacity. In Figure 1 tube I actsas a shunt capacity, In Figure 4 the phase shifting elements 0'2 and R'2 provide on the grid II a voltage which lags the generated voltage on the anode 22 and tube I acts as an effective inductance in shunt to the generator tank circuit II.

In both of the modifications shown in Figures 3 and 4 the modulating potentials are applied in push-pull relation as pointed out in detail in connection with Figures 1, 1a and 2. 4

I claim:

1. In a wave length modulation system, a tuned circuit including inductance wherein high frequency wave energy the wave length of which is to be modulated flows, a pair of tubes each so having an anode, a cathode and a control grid electrode, means coupling the anodes of said tubes to one end of said inductance, a connection between the cathodes of said tubes and a point on said inductance, phase shifting elements coupling the control grid of one of said tubes to a point intermediate the ends of said inductance, similar phase shifting elements coupling the control grid of the other of said tubes to one end of said inductanceand means for modulating the impedances of said tubes in phase opposition in accordance with signal.

2. Means for controlling the tuning of a circuit including an inductance wherein alternating current flows by controlling a reactive component of said circuit including, a pair of electhe cathodes of said devices and a point on said inductance intermediate the terminals thereof whereby alternating current of like phase is impressed on the anodes of each of said devices, phase shifting elements coupling the control grid of one of said devices to a point on said insubstantially phase quadrature with respect to the alternating current on the anode of said other device, and means for differentially controlling the impedances of said device to thereby control the reactances produced therein which supplement the reactance of said first named circuit to control the tuning thereof.

MURRAY G. CROSBY. 

